1. Field of the Invention
The present invention concerns a method of forming a reversible color image used in a display/recording apparatus, for example, for a hard copy or display, as well as a device for forming a reversible color image for practicing the method.
2. Description of the Related Art
There has been a great demand for color hard copies at a reduced cost and a heat sensitive full color recording system disclosed in Japanese Patent Laid-Open No. Hei 6-24020 (1994) has attained a practical level in recent years.
The system uses a heat sensitive color recording material having heat sensitive recording materials of three colors in lamination in which heat energy required for color formation is increased in the order of yellow, magenta and cyan and forms a full color hard copy by conducting (1) color formation only for yellow by low temperature heating thermal recording and exposure with a light at a first wavelength to fix yellow, (2) color formation only for magenta by medium temperature heating thermal recording and exposure with a light at a second wavelength to fix magenta and (3) color formation only for cyan by high temperature heating thermal recording, successively.
With the popularization of such a heat sensitive color recording system, it is expected that a demand will be increased for a reversible color image recording system capable of forming and erasing color repeatedly. In Japanese Patent Laid-Open No. Hei 6-79970 (1994) disclosing a reversible heat sensitive color recording medium and a reversible heat sensitive color recording system, there is described the constitution of a reversible heat sensitive color recording medium and a display medium, capable of easily conducting color formation and color erasure only by heating in a reversible heat sensitive recording medium and a display medium that utilize reaction between a color former and a developer, capable of stably maintaining the state of color formation and the state of color erasure at a normal temperature and having a color erasing temperature lower than a color forming temperature.
Description is to be made for the color image forming system with reference to FIG. 3 and FIG. 6. FIG. 3 is a constitutional view of a reversible heat sensitive color recording medium having a plurality of recording layers.
The reversible heat sensitive color recording medium 7 comprises a reversible heat sensitive recording layer 2A (forming color A), a reversible heat sensitive recording layer 2B (forming color B) and a reversible heat sensitive recording layer 2C (forming color C) having different tones of formed color and color erasure starting temperatures from each other and laminated on a support layer 1. An intermediate resin layer 3 is disposed between the recording layers and a protection layer 4 is formed at the upper surface. FIGS. 6A, 6B and 6C show a relationship between the color formation starting temperature and the color erasure starting temperature for each of reversible heat sensitive recording layers 2A, 2B and 2C. In FIG. 6, the abscissa indicates the temperature, while the ordinate indicates the color formation density.
In the drawings, each of the solid line curves indicates a density change when the temperature is raised from a color erased state. For example, in the recording layer 2A, the density rises at temperature TA1 and reaches a color-formed state at a temperature higher than that. The temperature TA1 is defined as the color formation starting temperature of the recording layer 2A. In the same manner, the color formation starting temperatures for the recording layers B and C are TB1 and TC1, respectively.
Further, each of the dotted curves in the drawings shows the change of the density when the temperature of the recording layer in the color-formed state is raised from the room temperature. For example, in the recording layer 2A, the density lowers abruptly at TA2 to erase color. The temperature TA2 is defined as the color erasure starting temperature of the recording layer 2A. For the recording layers B and C, the color erasure starting temperatures are TB2 and TC2 respectively. For the recording layers 2A, 2B and 2C, the color formation starting temperature and color erasure starting temperature are different from each other and the region indicated by an arrow between the color formation starting temperature and the color erasure starting temperature, namely, a color erasing temperature region is displaced between each of the recording layers 2A, 2B and 2C.
Description is to be made for the image forming system of forming each of colors by using the recording medium described above.
The color of the recording layer in the color-formed state can be erased by giving a temperature in the color erasing temperature region. Accordingly, when the recording medium 7 in which a color is formed is temporarily heated at temperature T2, temperature T3 and temperature T4, followed by cooling, respectively, colors are erased in the recording layer 2C at temperature T2, in the recording layer 2B at temperature T3 and in the recording layer 2A at temperature T4 respectively, so that the entire recording medium 7 turns colorless to attain initialization.
When the initialized recording medium 7 is heated temporarily to temperature T1 higher than the color formation starting temperature TC1 for the recording layer 2C followed by cooling, since color formation occurs not only in the recording layer 2C but also in the recording layers 2A and 2B, the recording medium 7 shows a mixed color of color A, color B and color C in three layers. That is, the mixed color of color A, color B and color C in the three layers can be obtained in the recording medium 7 by giving the temperature in the order of T2, T3, T4, T1.
When the initialized recording medium 7 is temporarily heated to temperature T2 higher than the color formation starting temperature TB1 for the recording layer 2B but lower than the color formation starting temperature TC1 for the recording layer 2C followed by cooling, color formation occurs in the recording layer 2B and the recording layer 2A, and the recording medium 7 shows a mixed color of color A and color B in two layers. That is, the color A and the color B in the two layers can be obtained in the recording medium 7 by giving the temperature in the order of T2, T3, T4, T2.
Further, when the recording medium 7 in which the mixed color in the three layers (A, B, C) is formed is temporarily heated to temperature T3 within the color erasing temperature region for the recording layer 2B (TA1&lt;T3&lt;TC2), followed by cooling, color erasure occurs in the recording layer 2B at temperature T3. However, since T3 is higher than the color formation starting temperature TA1 for the recording layer 2A and lower than the color erasure starting temperature TC2 for the recording layer 2C, no color erasure occurs in the recording layers 2A and 2C. Therefore, when the recording medium 7 in which the mixed color (A, B, C) is formed is temporarily heated to temperature T3, a mixed color (A, C) is obtained. That is, the mixed color of color A and color C in the two layers can be obtained in the recording medium 7 by giving the temperature in the order of T2, T3, T4, T1, T3.
When the recording medium 7 in which the mixed color (A, B, C) in the three layers is formed is temporarily heated to temperature T4 within the color erasing temperature region for the recording layer 2A (T4&lt;TB2), followed by cooling, color erasure occurs in the recording layer 2A at temperature T4. However, since T4 is lower than the color erasure starting temperatures TB2 and TC2 of the recording layers 2B and 2C, no color erasure occurs for the recording layers 2B and 2C. Accordingly, when the recording medium 7 in which the mixed color (A, B, C) is formed is temporarily heated to temperature T3, a mixed color (B, C) is obtained. That is, the mixed color of color B and color C in the two layers can be obtained in the recording medium 7 by giving the temperature in the order of T2, T3, T4, T1, T4.
Further, when the recording medium 7 in which the mixed color (A, C) is formed is temporarily heated to temperature T4 for the color erasing temperature region of the recording layer 2A (TA2&lt;T4&lt;TB2), followed by cooling, color erasure occurs only in the recording layer 2A and the recording layer 2C is kept as it is in the state of color formation, so that the color of the recording medium 7 is only the color of the recording color 2C. That is, the color C can be obtained for the recording layer 7 by giving the temperature in the order of T2, T3, T4, T1, T3, T4.
When the recording medium 7 in which a mixed color (A, B) is formed is temporarily heated to temperature T4 within the color erasing temperature region of the recording layer 2A (TA2&lt;T4&lt;TB2), followed by cooling, color erasure occurs only in the recording layer 2A and color formation is kept as it is for the recording layer 2B, so that the color of the recording medium 7 is the color only for the recording layer 2B. That is, the color B is obtained for the recording medium 7 by giving the temperature in the order of T2, T3, T4, T2, T4.
Further, when the initialized recording medium 7 is temporarily heated to temperature T3 higher than the color formation starting temperature TA1 for the recording layer 2A and lower than the color formation starting temperature TB1 for the recording layer 2B, followed by cooling, color is formed only for the recording layer 2A and the color of the recording medium 7 is color A. That is, the color A can be obtained in the recording medium 7 by giving the temperature in the order of T2, T3, T4, T3.
Accordingly, the relationship between the colors obtained and the temperature to be set successively for the method of forming the image is as shown in Table 1.
TABLE 1 __________________________________________________________________________ Color obtained Step 1 2 3 4 5 6 7 __________________________________________________________________________ Initialization T2 .fwdarw. T3 .fwdarw. T4 (colorless) Mixed color (A, B, C) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T1 Mixed color (A, B) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T2 Mixed color (A, C) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T1 .fwdarw. T3 Mixed color (B, C) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T1 .fwdarw. T4 Mono-color (C) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T1 .fwdarw. T3 .fwdarw. T4 Mono-color (B) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T2 .fwdarw. T4 Mono-color (A) T2 .fwdarw. T3 .fwdarw. T4 .fwdarw. T3 Background (colorless) T2 .fwdarw. T3 .fwdarw. T4 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. Entire Imagewise Formed/ Color color color erased erasure erasure formation color mixed +A +A +A -A +B +B -B +C __________________________________________________________________________
In the foregoing explanation, the heating temperature means a temperature at which heating is conducted temporarily. For example, heating from temperature T1 to temperature T3 means that the medium is temporarily heated to temperature T1, followed by cooling and then temporarily heated to temperature T3, followed by cooling again.
As can be seen also in Table 1, for forming an image using all the colors, three steps for initialization and four steps for image formation, namely, 7 steps in all are necessary. The image forming device for this steps comprises three heat rollers for heating the entire surface of a reversible heat sensitive recording medium at temperatures different from each other, and three thermal heads for heating at temperatures different from each other corresponding to the pattern of the image.
Although explanation has been made in the foregoing example to a case where the reversible heat sensitive recording medium layer 2 comprises three layers, color recording can also be conducted in the same manner also in a case for two layers or four or more layers. In a case of the two layers, an image of three colors, that is, a mixed color and two mono-colors can be obtained. Images of seven colors are obtained in the three layers and images of 15 colors can be obtained in the four layers respectively. When the colors are erased in each of the color-formed recording layers successively from the recording layer 2 having higher color erasure starting temperature, to conduct color erasure for the entire color recording layers, initialized state can be attained. Further, the recording medium 7 can form the color image repeatedly by this procedures.
FIG. 12 shows a fundamental constitution of a recording device for forming an image by the image forming system described above for the recording medium 7 having three reversible recording layers as described above.
Heat generating bodies L1, L2 and L3 are initializing heat generating bodies for heating the recording layers 2A, 2B and 2C entirely to respective color erasing temperatures thereby conducting color erasure over the entire surface. The heat generating bodies H1, H2 and H3 are image forming heat generating bodies for imagewise heating each of the recording layers 2A, 2B and 2C. When the color erasure starting temperatures for the recording layers 2A, 2B and 2C of the recording medium 7 are as shown in FIG. 6, each of the heat generating bodies L1, L2 and L3 is set such that the recording layer 2 can be heated successively to temperatures T2, T3, T4 and, if a multi-color image has already been formed to the recording medium 7, color erasure is conducted for the entire recording layers 2 in the recording medium 7 by passing the medium through the portion.
Subsequently, the recording medium 7 passes through the image forming section comprising the heat generating bodies H1, H2 and H3 capable of applying heating imagewise to the recording medium 7. Among them, the heat generating body H1 is used for color formation which generates heat so as to apply heating imagewise as required to temperature T1, T2 or T3. Succeeding heat generating bodies H2 and H3 generate heat imagewise as required such that the recording layer 2 can be heated to temperatures T3 and T4 respectively, thereby erasing a desired color in the recording layer 2 that forms a color by heat generating body H1.
More specifically, if only the recording layer 2C in FIG. 3 is a necessary portion, it is set such that the heat generating body H1 can be heated to temperature T1 and the heat generating bodies H2 and H3 can be heated to temperatures T3 and T4 respectively. When the medium passes through the heat generating body H1, the entire recording layers 2A, 2B and 2C form colors, whereas the recording layer 2B erases color upon passage through the heat generating body H2 and the recording layer 2A erases color upon passage through the heat generating body H3, so that the image area has a color only of the recording layer 2C.
Table 2 collectively shows such steps of forming the image and image formation is conducted by at 6 steps of heating for color formation only of the recording layer 2C.
A series of this process is the same as that noted for the formation of the image region of the background (colorless) in for the image region of the mono-color (C) in Table 1. In step 1, 2, 3, 4, 6 or 7, heating to one of the regions is applied. On the other hand, at step 5, heating is applied to none of the steps. Accordingly, it requires six steps of heating.
TABLE 2 ______________________________________ Heating Heating Heating device temperature region Process ______________________________________ 1 L1 T2 Entire C layer initialized surface (C layer erased) 2 L2 T3 Entire B layer initialized surface (B layer erased) 3 L3 T4 Entire A layer initialized surface (A layer erased) 4 H1 T1 Imagewise A, B, C layers image- (image area) formed (A, B, C layer image areas colored) 5 H2 T3 Entire B layer initialized surface (B layer erased) 6 H3 T4 Entire A layer initialized surface (A layer erased) ______________________________________
However, in the image forming system of the prior art, since colors of a plurality kinds of reversible heat sensitive recording materials are successively erased for initialization by entirely heating the recording medium or the display medium to the color erasing temperatures of the plurality kinds of the reversible heat sensitive recording materials, a number of processes are necessary for initialization and the recording medium or the display medium has to be heated over and over during the process, to result in a drawback of requiring a long time for initialization and low efficiency from an energy point of view.
Further, since the method of forming the image on the recording medium or the display medium requires a process for erasing another color formed simultaneously after forming the image of a mono-color, it results in a problem that the image forming process is further complicated than the erasure and the efficiently is poor in view of time or energy.
Furthermore, since for conducting such complicate processes, it is necessary in the image forming device to provide a plurality of heating devices for heating the entire recording medium or display medium to color erasing temperatures respectively, it suffers from a drawback that the entire device is complicated in the constitution and enlarged in the scale.